Project Summary: Temporal lobe epilepsy (TLE) develops in a third of over 300,000 patients with a first seizure and over 30% of cases are resistant to drugs contributing to a significant disability. Presence of a therapeutic time window between the initial insult and development of epilepsy suggests that improved mechanistic understanding of early pathological process may enable prevention of epileptogenesis and associated co-morbidities. While sclerosis of the hippocampal dentate gyrus characterizes late stage TLE, cell loss, network reorganization and deficient inhibition in the dentate gyrus occur soon after insults that progress to TLE. In particular, the dentate inhibitory gate which limits GC activity throughput is compromised early in acquired TLE. However, what cells and circuits make up the dentate inhibitory gate and how this is compromised after seizures is not fully understood. Recently, a new class of neurons, semilunar granule cells (SGCs) were proposed as drivers of sustained dentate feedback inhibition. Although SGC-like neuros are observed in multiple species including humans and are activated during behaviors, the development, molecular identity, and connectivity of SGCs are not known making it difficult to determine their role in dentate function and disease. The limited literature and our pilot data that SGCs input and output connections are distinct from granule cells indicating that they play a unique role in dentate processing. This study will test the hypothesis that SGCs from a parallel dentate circuit that strengthens inhibition in the normal brain. We further propose that cellular and network changes after seizures compromise SGC mediated inhibition and augment their excitatory effects contributing to epilepsy and memory deficits. Combining morphometry, Patch-seq transcriptomics, electro- and optophysiology in transgenic mouse lines subject to experimental epilepsy and computational modeling will allow us to test the above hypothesis. Aim 1 will define the cellular and circuit identity of SGCs and determine molecular markers. Aim 2 will determine if the SGC excitatory circuit is strengthened and feedback inhibitory circuit compromised after status epilepticus. Finally, Aim 3 will examine the normal and seizure-induced development of SGCs and their contribution to dentate memory processing. On completion the studies will eliminate specific knowledge gaps in how the dentate circuit functions in behaviors and epilepsy, in keeping with the NINDS mission, and provide information needed to prevent collapse of dentate inhibition soon after seizures and prevent development of epilepsy and memory co-morbidities.